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Tin-Free Access to the ABC Core of the Calyciphylline A Alkaloids and Unexpected Formation of a D‑Ring-Contracted Tetracyclic Core

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journal contribution
posted on 03.04.2018, 15:19 by Alberto M. Lopez, Ahmad A. Ibrahim, Gregory J. Rosenhauer, Hansamali S. Sirinimal, Jennifer L. Stockdill
A tin-free strategy for the successful cyclization of a variety of internal alkyne-containing N-chloroamine precursors to the ABC core via cyclization of a neutral aminyl radical is established. Deuterium labeling experiments confirm that the solvent is the primary source of the final H atom in the cyclization cascade. These conditions enabled a streamlined route to a β-ketoester intermediate poised for intramolecular Knoevenagel condensation to construct the seven-membered D-ring of calyciphylline A alkaloids. However, exposure to CsF in t-BuOH at elevated temperatures led to an unexpected decarboxylation to form a D-ring-contracted tetracyclic core.